The direction of “‘up’ can be measured by judging the orientation of a line against gravitational vertical: the subjective visual vertical (SVV). An alternative method uses the perceived identity of a character whose identity depends on its orientation (the Oriented CHAracter Recognition Test (Dyde et al., 2006: Exp Brain Res. 173: 612). OCHART derives the perceptual upright (PU) from the average of the two orientations at which the character is maximally ambiguous. PU and SVV are differently influenced by the directions of gravity, the body and visual cues. However SVV is derived from a point of maximum certainty whereas PU is taken from points of maximum uncertainty. Could different results arise from differing methodologies?

Methods

SVV was measured in two ways, both using constant stimuli. The conventional method determined an orientation of maximum certainty (“was the line tilted clockwise or counterclockwise relative to gravity?”). The new method found two points of maximum uncertainty (making it more comparable with OCHART) by asking whether a tilted line appeared closer to vertical or horizontal - the average of these two orientations indicating another SVV. For eight observers both SVVs were calculated whilst placed against a background rich in polarity cues which was tilted through 360° in 22.5° steps.

Results

Both SVVs showed a complex but clearly similar influence of the visual background's orientation and the methods showed indistinguishable results when the background was either fully upright or fully inverted - both being within 1° of gravitational vertical. However the SVV derived from the two points of maximum uncertainty showed a significantly larger influence of the background and higher intra-observer variances.

Conclusions

Both methods generated comparable results, but have differing sensitivities to tilting the visual background with the reliability of the estimate of the orientation of the probe relating to the magnitude of the background effect.

Supported by NASA Cooperative Agreement NCC9-58 with the National Space Bio-medical Research Institute and grants from the Natural Sciences and Engineering Research Council of Canada and the Canadian Space Agency (CSA).